The transmission-type liquid crystal display (LCD) exhibits a high contrast ratio and good color saturation. However, its power consumption is high due to the need of a backlight. At bright ambient, the display could be washed out completely. On the other hand, a reflective LCD is using ambient light for reading displayed images. Since it does not require a backlight, its power consumption is reduced significantly.
Reflective cholesteric liquid crystal display (Ch-LCD) is a bistable device. Once the LC directors are reoriented, they stay. Thus, Ch-LCD consumes less power than the general reflective twisted nematic (TN) LCD, super-twisted nematic (STN) LCD or thin film transistor (TFT) LCD. Due to its bistability, the driving voltage is required only when a user wants to refresh the screen. This power-saving feature is especially important for reading books or magazines. An ordinary person may take 2–3 minutes to finish reading a page. Thus, Ch-LCD is a strong contender for electronic newspaper or books.
The operating principle of reflective cholesteric display is shown in FIG. 1(a) and FIG. 1(b). FIG. 1(a) shows the bright state of a Ch-LCD wherein the cholesteric LC molecules 10 are arranged in layers with the helical axes perpendicular to top substrate 17 and bottom substrate 18. When an unpolarized light 11 is incident to a right-hand cholesteric LC layer 12, the right-hand circularly polarized light 13 within the bandwidth is reflected and the transmitted left-hand circularly polarized light 14 is absorbed by the absorption layer 15, which can be black paint. In a high voltage state as shown in FIG. 1(b), the cholesteric LC layer was driven into a focal conic state 16 wherein the LC molecules are almost aligned with the helical axes parallel to top substrate 17 and bottom substrate 18. Thus, the incident light passes through the LC layer and is absorbed by the absorption layer 15, resulting in a dark state.
However, at dark ambient, a reflective LCD loses its visibility. To enable a reflective display to be usable for dark ambient, a transflective display has been invented. In a transflective display, illustrated in FIG. 2, each pixel 20, having a single cell gap 25, is divided into transmissive 21 and reflective 22 portions, sometimes called sub-pixels. The transflective display is the most versatile because it works great in well-lighted and poorly lighted environments. However, cholesteric displays are not workable on such transflective structure. As FIG. 3(a) shows, unpolarized light 30 is reflected 32 and unpolarized backlight 31 is transmitted 33 in each sub-pixel, resulting in a bright state when no voltage is applied. Such display lacks a dark state. In FIG. 3(b), the ambient light 34 is absorbed in the reflective portion 35 of the sub-pixel. However, the backlight 36 transmits through the transmissive portion 37 of the sub-pixel. Thus, the transmissive portion has no dark state, with the voltage on or off.
Cholesteric liquid crystal is known to possess memory effects. Thus, its power consumption is much lower than the TN LCD or STN LCD. Table 1 summarizes the calculated battery time of a video graphics array (VGA), 6.3-inch diagonal full color display for different display technologies. Obviously, the cholesteric display offers a significant power saving over the STN and active matrix TFT LCDs. When considering an average reading time of one minute per page, the cholesteric LCD of the present invention provides more than 370 additional hours of display time between battery recharges when compared to a bistable reflective cholesteric display with passive matrix addressing.
Table 1. Calculated battery life of a VGA, 6.3-inch diagonal full color display in terms of the battery is 5.4 Watt-hours lifetime or operating time between battery recharges for different display technologies.
Time Between Recharges forVarious Average Reading Times125Display Technologiesmin/pagemin/pagemin/pageRefreshed type display with backlight 2 hrs 2 hrs  2 hrssuch as an STN or Active Matrix TN.Refreshed type reflective display with 18 hrs 18 hrs 18 hrssmart electronics.Bistable reflective Cholesteric display270 hrs540 hrs1350 hrspassive matrix addressing.Bistable reflective Cholesteric display640 hrs1280 hrs 3200 hrsactive matrix addressing 50% pixelchange.
Various prior art references related to transflective cholesteric displays are found. The two published papers are: 1. International Display Workshop (2001), p. 129, by Yuzo Hisatake et al, (Toshiba), and 2. SID'00, p. 742, by Rob van Asselt et al (Philips). The device structures are shown in FIG. 4(a) through FIG. 4(f), respectively. The display device from Philips includes a polarizer 40, a retardation film 41, a non-twist LC layer 42a (with voltage off) and 42b (with voltage on), a cholesteric transflector 43 and absorption layer 44. In reflective mode, when no voltage is applied, a bright state will occur as shown in FIG. 4(a); while under applied voltage, as shown in FIG. 4(b), the phase retardation of non-twist LC layer 42b changes half-wave. Therefore, the circularly polarized light changes twist sense accordingly and passes through the cholesteric transflector 43 and is finally absorbed by the absorption layer 44, resulting in a dark state. FIG. 4(c) shows the normally white image 45a in reflective mode. In transmissive mode, when no voltage is applied, non-twist LC layer 48a remains flat, no light can pass through polarizer 46 and a dark state occurs, as shown in FIG. 4(d). Under applied voltage, as shown in FIG. 4(e), the linearly polarized light changes 90° since phase retardation of non-twist LC layer 48b changes half-wave, therefore the linearly polarized light passes through polarizer 46, resulting in a bright state. FIG. 4(f) shows the normally black image 45b in transmissive mode. The major difference between Philips' display and previous cholesteric displays, as shown in FIGS. 1(a) and 1(b), is that in Philips' display, the cholesteric layer is used as a transflective reflector, no voltage is applied to switch the cholesteric layer. In previous cholesteric display references, the cholesteric layer is used as a light switch.
Three issued patents are found related to the full color cholesteric displays. The first one is U.S. Pat. No. 6,377,321 in which a full color cholesteric display was fabricated by stacking three cells of primary RGB (Red, Green and Blue) colors. See FIG. 5(a). The second is U.S. Pat. No. 6,061,107 in which different UV intensity is used to generate RGB pixels with different pitch lengths (FIG. 5(b)), and the third is U.S. Pat. No. 5,949,513 in which different twist agents are used to generate RGB color pixels (FIG. 5(c)).
In the panel stacking system 50 as shown in FIG. 5(a), parallax problems will occur because of the reflective image from three different stacking layers when viewed from an oblique direction. This parallax would greatly limit the device resolution. To reduce parallax, the substrate thickness has to be reduced to less than 0.3 mm, which can only be achievable by using plastic substrates. Additionally, pixel registration is another concern. These additional steps will undoubtedly increase the fabrication time and cost. FIG. 5(b) shows the prior art using different UV intensity 51 to generate red, green, blue (RGB) pitch lengths. The three primary color pixels need to be cured by different UV intensity. This is a rather complicated process and cannot be done in a single mask. FIG. 5(c) shows the method of using different twist agent doping. As the drawing shows, there is a need for wells 52, 53, 54, 55 in order to separate three primary color regions and to deposit two different twist agents 56, 57.
A common drawback of the above-mentioned prior art devices is that they are reflective displays, i.e., they need ambient light to read the displayed information contents. In a dark ambient, these displays are not readable. Even if it can achieve a transflective display, the contrast ratio at reflective mode and transmissive mode is reversed, which will actually decrease the contrast ratio if both modes work simultaneously.